1. Field of the Invention
The present invention relates to a new and improved method of measuring the impedance of a sensor by applying a sine-shaped AC voltage of defined frequency and to a circuitry for carrying out this method. This type of impedance measurement is especially important for pure AC - measurements as is necessary e.g. to determine the resistance of electrolytic solutions.
2. Description of the Prior Art
A conventional method of measuring impedances by a sine-shaped AC voltage is disclosed in H. Helke's "Messbrucken und Kompensatoren fur Wechselstrom", R. Oldenburg Verlag, 1971, p. 243, and is based on a lock-in amplifier for variable frequency. A prior art circuitry of this type is shown in annexed FIG. 1. The sine-shaped AC voltage is applied to a measuring circuit containing the impedance Z.sub.x to be measured, the output of which then is supplied to a rectifier circuit controlled by a square wave AC voltage. In order to adjust the phases of said two AC voltages, which is a necessary condition for the proper function of the circuitry, the phase of the square wave AC voltage is shifted.
Another possibility for adjusting said phases which has been developed by the inventor of the present invention in order to avoid said phase shifting consists in using a filter circuit connected at the output of the measuring circuit containing the impedance Z.sub.x as shown in FIG. 2 which replaces the above phase shift. By appropriate construction of the filter circuit only the AC voltage proportional to the impedance to be measured is phaseshifted at its output (but not all other interfering frequencies) such that it is in phase with the square wave AC voltage. The present invention refers to this type of measuring circuit.
A circuit known from Electronics & Power, Vol. 23, No. 5, May 1977, p. 387, also uses a filter circuit connected to the measuring circuit containing the impedance. This filter circuit, however, is used to separate the measured signal from the interfering frequencies and not to establish a given phase relationship between the sine- and square waves.
In the publication FR-A-2 049 584 a circuitry is disclosed which is used as a rectifier or a modulator. In this circuit a resistor is grounded by means of a square wave, but this circuit does not incorporate any low-pass circuit.
In all the above circuitries, however, there is the problem of linearity when measuring extremely low impedances, the measured voltages being in the range of millivolts.